See Also

Lesson Learned: Measuring Fret Markings

Fret markings make playing a fretless and raised string instruments MUCH easier, by the way, without, of course, affecting the sound of the instrument. It's common for lap steel guitars to use actual frets as markers; in my case, I actually made marks on the instrument.

For a twelve-tone equal temperament (this is the kind of tuning that is used in Western music, by and large), it works like this...

the distance between the nut and the fret 1 = distance from nut to bridge
(1/17.835 * distance from nut to bridge)

the distance between fret 1 and fret 2 = distance from fret 1 to bridge
(1/17.835 * distance from fret 1 to bridge)

and so forth...

The measure/mark technique is sometimes described as employing the "rule of 18," since using a rounded-off value of 18 in place of 17.835 yields pretty decent results and builds in some correction for string height on fretted instruments. Some luthiers use the figure 17.817 instead.